All depends on how you define "unsuccessful" in this context. The category description says:
"Nominees to the Supreme Court of the United States have been unsuccessful for a variety of reasons. In some cases, the Senate has voted down the nominee, or even refused to hold a hearing on the nomination. In other cases, the nominee declined the nomination or the nomination was withdrawn."
Given that last sentence, "unsuccessful" doesn't just mean the nomination was not approved by the Senate. One could argue that his nomination was unsuccessful at resulting in his being seated because death intervened. (Just sayin'.)
If y'all decide to keep the category, you could add to the definition "or the candidate died before taking their seat."
Or not.